The philosophical underpinning is giving up of materialness. The practicality of the 5 instances that I witnessed over the past year - typical terminal individuals choose this. They pass away surrounded by loved ones (they typically medicate for any pain, and the body starts shutting down when food and water stops). This is observed with somberness, but celebrated as very positive act.
When someone starts this process, it's a unique experience speaking with them, as there's usually nothing that comes up, and the moment does not really lend itself to small talk :)